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<h1 style="direction:ltr;"><b><font color="#808000">Overview</b></font></h1>
<ul><li style="direction:ltr;"><h2>Congestion control vs. flow control</h2></li>
<ul><li style="direction:ltr;">Congestion control: effectively utilize the network bandwidth</li>
<li style="direction:ltr;">Flow control: prevent the receiver from being overwhelmed by incoming packets</li>
</ul><li style="direction:ltr;"><h2>Window-based vs. rate-based</h2></li>
<ul><li style="direction:ltr;">Window-based: tune the maximum number of on-flight packets (TCP)</li>
<li style="direction:ltr;">Rate-based: tune the inter-packet sending time (UDT)</li>
</ul><li style="direction:ltr;"><h2>AIMD: additive increases multiplicative decreases</h2></li>
<li style="direction:ltr;"><h2>Feedback</h2></li>
<ul><li style="direction:ltr;">Packet loss (Most TCP variants, UDT)</li>
<li style="direction:ltr;">Delay (Vegas, FAST)</li>
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<h3>Notes:</h3>
<p style="direction:ltr;"><font color="#000000">Before we go into the details, let me clarify several issues about UDT's congestion control algorithm.</font></p>
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<p style="direction:ltr;"><font color="#000000">When we say &quot;congestion control&quot; here, we mean the mechanism to effectively utilize the network bandwidth. Another term, &quot;flow control&quot;, is usually used to name the approach to prevent incoming traffic overwhelming the receiver. Congestion control is a more difficult problem.</font></p>
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<p style="direction:ltr;"><font color="#000000">Congestion control can tune the data sending rate in two ways: limit the number of on-flight packets and set the inter-packet time. TCP uses the window-based approach, whereas UDT uses the rate-based approach.</font></p>
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<p style="direction:ltr;"><font color="#000000">The most common congestion control algorithm is TCP's AIMD algorithm. We have already discussed AIMD.</font></p>
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<p style="direction:ltr;"><font color="#000000">Another important factor in congestion control is how to indicate a network congestion. Most protocols uses packet loss information, including both TCP and UDT. The increase in round trip time delay can also indicate congestion somewhere along the network.</font></p>
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